1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which stores “1”- and “0”-data using a magnetoresistive effect.
2. Description of the Related Art
In recent years, many memories which store data by new principles have been proposed. One of them is a magnetic random access memory which stores “1”- and “0”-data using a tunneling magnetoresistive (to be referred to as TMR hereinafter) effect.
As a proposal for a magnetic random access memory, for example, Roy Scheuerlein et al, “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest, p. 128 is known.
A magnetic random access memory stores “1”- and “0”-data using TMR elements. As the basic structure of a TMR element, an insulating layer (tunneling barrier) is sandwiched between two magnetic layers (ferromagnetic layers).
Data stored in the TMR element is determined on the basis of whether the magnetizing states of the two magnetic layers are parallel or anti-parallel. “Parallel” means that the two magnetic layers have the same magnetizing direction. “Anti-parallel” means that the two magnetic layers have opposite magnetizing directions.
Normally, one (fixed layer) of the two magnetic layers has an anti-ferromagnetic layer. The anti-ferromagnetic layer serves as a member for fixing the magnetizing direction of the fixed layer. In fact, data (“1” or “0”) stored in the TMR element is determined by the magnetizing direction of the other (free layer) of the two magnetic layers.
When the magnetizing states in the TMR element are parallel, the resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers of the TMR element is minimized. For example, this state is defined as a “1”-state. When the magnetizing states in the TMR element are anti-parallel, the resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers of the TMR element is maximized. For example, this state is defined as a “0”-state.
Currently, various kinds of cell array structures have been examined for a magnetic random access memory from the viewpoint of increasing the memory capacity or stabilizing write/read operation.
For example, currently, a cell array structure in which one memory cell is formed from one MOS transistor and one TMR element (or an MTJ (Magnetic Tunnel Junction) element) is known. Additionally, a magnetic random access memory which has such a cell array structure and stores 1-bit data using two memory cell arrays so as to realize stable read operation is also known.
However, in these magnetic random access memories, it is difficult to increase the memory capacity. This is because one MOS transistor corresponds to one TMR element in these cell array structures.